Aqueous emulsion polymers, also called latexes or polymer dispersions, are known and used in both clear and pigmented form. They are used in many products, including interior and exterior architectural coatings, general metal coatings, adhesives, and the like. These latexes can be formed by aqueous emulsion polymerization of ethylenically unsaturated monomers such as styrene and its derivatives; acrylic and methacrylic acids; alkyl acrylates, methacrylates, and hydroxyl-substituted derivatives; vinyl acetate; acrylonitrile; glycidyl acrylates and methacrylates; and the like.
The large number of independent variables which can be manipulated in designing latex particles renders the preparation of improved latexes technically challenging. The emulsion polymers used in aqueous coating compositions must be hard enough to resist physical and chemical forces, but, at the same time, soft enough to form a continuous film. Film durability, water resistance, and chemical resistance are provided by hard polymers with glass transition temperatures (Tg) above ambient temperature. However, to be film-forming, these polymer dispersions must have a minimum filming temperature (MFT) at or below ambient temperature, to allow for fusion of the polymer particles into a continuous film.
Film properties can be further improved if the polymer film crosslinks upon film formation.
The preparation of emulsion polymers containing “dangling” or pendant double bonds is one possible solution for a coating system which crosslinks ambiently, thermally, and photochemically. See, for example, U.S. Pat. No. 5,539,073.
U.S. Pat. No. 5,596,035 discloses autocrossslinking aqueous dispersions, having a minimum filming temperature (MFT) of from 0° C. to about 50° C., which are a mixture of at least one carbonyl-containing soft latex polymer having an MFT of below about 20° C., at least one hard latex polymer having an MFT of above about 25° C., and at least one polyfunctional carboxylic hydrazide.
U.S. Pat. No. 6,005,042 discloses a polymer dispersion prepared by means of stepwise emulsion polymerization of monomer mixtures which include a proportion of hard monomers of at least 30% by weight in the first polymerization stage and at least 65% by weight in the second polymerization stage. The dispersions are said to have an MFT in the range from 0° to 40° C., and to form polymer films which possess high blocking resistance and scratch resistance with sufficient elasticity for the coating of substrates that are not dimensionally stable. To increase the chemical resistance, the dispersion may also include polyfunctional carboxylic hydrazides.
WO 98/08882 discloses an aqueous two-stage emulsion polymer having a first stage polymer with a calculated glass transition temperature of at least 70° C. and a second stage polymer having a calculated glass transition temperature between about 5° to 50° C., with a wet-adhesion promoting monomer being used in the second stage polymer.
EP 1 149 875 discloses an aqueous stain-blocking coating composition including an aqueous emulsion copolymer having a glass transition temperature (Tg) from −20 to 60° C. and including, as polymerized units, at least one ethylenically unsaturated nonionic monomer and 1.5-6%, by weight based on the dry weight of the copolymer, ethylenically unsaturated strong acid monomer, such as a phosphorus-containing strong acid monomer, or salts thereof.
EP 1 302 515 discloses a coating composition comprising a bimodal aqueous emulsion copolymer that includes as polymerized units from 0.1% to 10% by weight, based on the dry weight of the copolymer, of a strong acid monomer, or salts thereof. The bimodal copolymer contains small mode particles having a diameter of 50 to 150 nm, and large mode particles having a diameter of less than 400 nm. The coating composition is said to be particularly useful as a stain blocking coating.
While multi-stage emulsion polymers have been described in the prior art, there is a continuing need for aqueous dispersions containing emulsion polymers which provide a coating or film having excellent performance properties such as blocking resistance at elevated temperature, high gloss, water and chemical resistance, and excellent low temperature film formation, all with minimal VOC levels.